A theoretical study on the structure of thiazolidine-2,4-dione and its 5-substituted derivatives in the gas phase. Implications for the thiazolidine-2,4-dione -water complex

The results of a detailed DFT (B3LYP) investigation on five tautomers of thiazolidine-2,4-dione and their 5-substituted derivatives (CH3, NH2, Cl, F, CN- and NO2) are presented here. The energy, geometrical parameters, topological parameters of all species in the gas phase have been calculated at B3LYP6–311++G(3df,2p)//B3LYP/311 + G(d,p) level of theory. The proton affinities (Pas), molecular electrostatic potential (MEP), natural valence atomic orbital energies (NNAO) of the basic center exist in the title compound in the gas phase have been calculated at the same level of theory. The specific hydration of the title compound by one water molecule has been also investigated at the same level of theory. Among the thiazolidine-2,4-dione tautomers and its derivatives, the most stable tautomer corresponds to the diketo forms (A), regardless of the substituent type. Results reveal that the oxygen atom of the carbonyl group at position 2 is more basic than the one at position 4. The existence of different hydrogen bond donor and acceptor centers in these molecules led to different kinds of intermolecular hydrogen bonds (CH···O, NH···O and CO···H) and different kinds of complexes. The stability of the cyclic complexes has been investigated using the analysis of Natural Bond Orbital (NBO), Atoms In Molecules topology, and the thermodynamic data. Results suggest that the water molecule prefers to bind with the oxygen atom, which has low intrinsic character.